Electronic substrates such as printed wiring boards and interconnects often comprise plated vias and thru-holes. These holes (the terms “hole” and “holes” will often be used herein to refer to both vias and thru-holes if the text is applicable to both vias and thru-holes) are often filled with a conductive paste or other fill material, typically by forcing the material into the hole from one end of the hole. For the sake of this disclosure, the end of a hole through which fill material enters the hole will be referred to as the “entrance”, and the opposite end of the hole as the “exit”.
When filling holes, it is important that the exits of the holes not be in contact with any surface that will later be removed from the substrate before the fill material sets or cures. Thus, if one side of the substrate is resting on a tooling plate during the fill process, there should be some separation between the tooling plate and the exit of the hole in order to prevent material drag-out as the substrate is removed from the tooling plate.
However, such separation is not always easy to achieve. Substrates tend to be relatively flexible, and the filling process often exerts a downward force on the substrate. Thus, any mechanism used to provide the required separation must be adequate to maintain that separation while pressure is applied to the substrate. Providing proper separation is made more difficult because of the need to be able to handle a large number of different hole patterns as utilizing a single stand-off for multiple hole-patterns runs the risk of having portions of the stand-off intended to support the substrate positioned directly beneath a hole to be filled.
One method of supporting a substrate during filling is to provide a tooling plate or other support having through holes formed in it, placing the substrate to be filled on top of such a support in a manner that the holes to be filled align with through holes of the support, and filling the through holes. Examples of such supports and their use can be found in U.S. Pat. No. 6,276,055, U.S. Pat. No. 6,282,782, and GB2246912. Unfortunately, known supports tend to be inadequate in applications where a substrate comprises a larger number of holes having minimum separation between holes such as in a grid-array arrangement. In such an application, one might simply remove an entire area of the support corresponding to the area containing the holes, but doing so would leave that area of the substrate unsupported. Moreover, even if the support comprises individual holes rather than a removed area, there is a chance that the support will distort under the pressures applied during filling with a corresponding misalignment between the through holes of the support and the holes of the substrate. Known supports are often relatively expensive to manufacture as well.
Thus, there is a continuing need for improved methods and devices to facilitate hole filling.